1. Field of the Invention
This invention relates to a navigation instrument for use with roadway vehicles such as rally cars, autobuses and trucks operated on a regular run, and so forth.
2. Description of the Prior Art
Navigation instruments have been incorporated in roadway vehicles for calculating the displaying vehicle running conditions with respect to scheduled ones so that the driver can arrive at the destination on time.
Typical conventional navigation instruments comprise a pulse generator providing a pulse every time the vehicle travels a constant distance, a first counter counting the pulses fed from the pulse generator, a clock pulse generator, a second counter counting the clock pulses fed from the clock pulse generator, an arithmetic circuit calculating the average speed of the vehicle, the deviation of the travelling distance from a scheduled distance, and the deviation of the travelling time from a scheduled time in accordance with the contents of the first and second counters and a scheduled average speed, and a display control circuit responsive to the calculated results to drive a display unit so that the calculated results can sequentially be displayed on the display unit.
With such conventional navigation instruments, however, the possibility exists that the reliability of the contents displayed on the display unit may be low during the initial running operation of the vehicle. There are several reasons for such lack of reliability. First, since the clock pulse generator is designed to generate clock pulses at an interval increasing with the vehicle speed decreasing so as to meet the counting capacity of the counter at high vehicle speeds, the number of the pulses fed from the pulse generator is fewer with respect to the time interval of the pulses fed from the clock pulse generator during the initial running operation of the vehicle. This results in counting accuracy reduction. Second, the first and second counters are actuated at a desired time. If they are actuated at a time after the clock pulse generator provides one clock pulse and before it provides the next clock pulse, an error will be introduced in the data calculation performed during the time interval.
An attempt has been made to provide a zero-flashing or blanking indication for a predetermined time after the vehicle starts so as to serve notice on the driver that the accuracy of the contents to be displayed is insufficient. However, the possibility still exists that a great error is introduced in the displayed results if the vehicle is started with intermittent operations.